Encoders provide a measurement of the position of a component in a system relative to some predetermined reference point. Encoders are typically used to provide a closed-loop feedback system to a motor or other actuator. For example, a shaft encoder outputs a digital signal that indicates the position of the rotating shaft relative to some known reference position that is not moving. A linear encoder measures the distance between the present position of a moveable carriage and a reference position that is fixed with respect to the moveable carriage as the moveable carriage moves along a predetermined path.
Optical encoders utilize a light source and a photodetector to measure changes in the position of an encoding disk or strip. In a transmissive encoder, the encoding disk includes a series of alternating opaque and transparent strips. The light source is located on one side of the code strip, and the photodetector is located on the other side of the encoding strip. The light source and photodetector are fixed relative to one another, and the code strip moves between the photodetector such that the light reaching the photodetector is interrupted by the opaque regions of the code strip. The position of the code strip is determined by measuring the transitions between the light and dark regions observed by the photodiode.
In a reflective encoder, the light source and photodetector are located on the same side of the encoding strip, and the encoding strip consists of alternating reflective and absorbing stripes. The light source is positioned such that light from the light source is imaged into the detector when the light is reflected from the reflective strips.
Transmissive encoders have a number of advantages over reflective encoders in terms of tolerance and contrast ratios. Transmissive encoders are typically constructed from two separate sub-assemblies, a light source and a detector. In a transmissive encoder, the light from the light source is colliminated before it reaches the code strip, and hence, the light leaving the code strip is also colliminated. The light source is typically constructed from an LED and a collimating lens. The only critical distance is the distance from the lens to the LED, which can be tightly controlled by the manufacturer of the light source sub-assembly. The detection assembly needs only to image this colliminated light onto the detector surface. Hence, the only critical distance is the distance from the imaging lens to the detector, which can also be tightly controlled by the detector manufacturer independent of the specific encoder assembly. Furthermore, the same sub-assemblies can be utilized for a wide variety of encoders, since the distances between the light source and the code strip and between the code strip and the detector module are not critical.
In a reflective encoder, in contrast, the distance between the code strip and the detector is critical as either the code strip itself or the light source as seen in the reflected light from the code strip is imaged into the detector. However, reflective encoders have the advantage of requiring only one component, namely an emitter-detector module that includes the LED, photodetector, and one or more lenses. Hence, the manufacturer of the encoder needs to mount and align only one component. Furthermore, the encoder design does not have to provide space for the light source on the other side of the code strip. As the size of encoders is reduced in response to smaller mechanical systems, eliminating the need to mount components on both sides of the code strip becomes increasingly important.
Prior art emitter-detector modules for reflective encoders must be custom designed for each encoder design. The lenses within the emitter-detector module must be chosen to match the code disk resolution and size as well as the distance between the code strip and the emitter-detector module. In prior art designs, the light source and the photodetector are typically mounted on a substrate and then encapsulated in a clear resin. The top surface of the resin is molded to provide the lens functions. Hence, different emitter-detector modules require different molds. This reduces the economies of scale in the manufacture of the emitter-detector modules, and hence, increases the cost and product cycle times.